1. Field of the Invention
The present invention relates to a method for examining the metallic precipitate on grain boundary (GB) and heterogeneous grain growth (GG). Particularly, the present invention relates to a method for utilizing the partial etching using chemical plasma (less ion bombardment) and scanning electron-microscope (SEM) to observe the metallic precipitate easily and efficiently.
2. Description of the Prior Art
As bipolar technology advances, smaller sized devices and circuits with higher functional densities are implemented. In the course of integrated-circuit evolution, the maximum number of devices per chip has steadily increased while the number of the metal pitches has steadily decreased, mainly because of the increase in functional density. The metal pitches is defined as the summation of linewidth and the width between two neighboring metal lines. In the year of 1970s the metal pitch is about the order of 10 .mu.m, yet as the functional density raises, the metal pitch is down to the order below 1 .mu.m.
The density of a device of the most advanced circuits is limited by the area occupied by the interconnect paths. Anisotropic etching of the metal layers permits the use of smaller minimum metal pitches to increase the interconnect capability. Thus, the isotropic nature of aluminum wet etching processes renders them inadequate for VLSI applications, and it requires a direct dry etching process. The gas mixtures 10 (neglecting rare gas diluents) including BCl.sub.3 +Cl.sub.2 are used in the prior art to successfully etch the aluminum. The equipment used to implement the chemical plasma etching is shown in FIG. 1.
Some materials are added to the aluminum to improve some of its properties. For example 1-2% silicon is often added to prevent the aluminum from spiking near shallow junctions. In addition 2-4% copper or 0. 1-0.5% Ti(often together with Si) are usually added to enhance the electromigration resistance of the aluminum. Since SiCl.sub.4 is volatile at room temperature, Al--Si films are readily etchable in chlorine-containing gases. Titanium also forms volatile etch products (TiCl.sub.4) which dose not pose a problem. The copper, on the other hand, reacts with the chlorine to form an etching product CuCl, which is relatively non-volatile below 175.degree. C. The copper containing residues often remains after these alloy films were dry-etched, so it is more difficult to use the chlorine plasmas to perform the etch step. The degree of difficulty increases with the increased Cu concentration and 4% Cu-containing films being quite a severe challenge. The successive processes illustrating chemical plasma etching of the wafer without eliminating Cu precipitate are shown from FIG. 2(a) to FIG. 2(c). In FIG. 2(a), at the point 24 where segregation occurred in the Al--Cu alloy film 22, the etching rate is different from that of elsewhere in the Al--Cu alloy film 22.
FIG. 2(b) shows how the Cu precipitates are produced during the etching process, it also shows that some points possess different etching rate and act as a mask that is called as a micro masking which produces unwanted particles. FIG. 2(c) shows the end point of the chemical plasma etching and the foregoing conduction is called `bridging issue`, which causes short-circuit between the metal lines 28 and 29. Two methods are used to promote CuCl desorption during the chemical plasma etching: heating the substrate to the temperature commensurate with the maximum temperature that the resist material is allowed to be used, or enhancing the ionic bombardment of the surface so that the significant sputtering may occur. FIG. 3 is a cross-sectional view of a wafer after the enhanced chemical plasma etching with the copper precipitate eliminated. During the sputter deposition of Al--Cu alloy film, copper segregation occurred at the grain boundary of aluminum atoms, and the Cu precipitation resides after ion bombardment. The chemical plasma etching utilized to perform dry etching Al--Cu alloy film is intentionally set in the environment that produces ion bombardment so the copper precipitate is thus partly eliminated. After the etching process, the TEM is used to perform the quantitative analysis.
To perform the quantitative analysis, the method transmission electron microscopy (TEM) is often used. The TEM offers the maximum resolution of 2 angstrom. The image in the TEM is produced by the differential loss of electrons from an incident beam(60-350 keV, electron wavelength .about.0.04 angstrom)as it passes through very thin samples. The sample must be thin enough to transmit the beam so, that the essential information caused by differences in sample thickness, phase composition, crystal structure, and orientation is preserved. In a conventional TEM, the electron beam is focused by a condenser lens, then passes through the sample and is imaged onto a photographic plate or fluorescent screen. The contrast in a TEM image arises for different reasons in samples of crystalline and amorphous materials. In crystalline layers, the incident electron beam is diffracted by the material. Abrupt changes in thickness, path structure, or crystallographic orientation to cause corresponding changes in contrast and these crystallographic feature can be easily imaged at high resolution. In amorphous regions, contrast is obtained from samples of different thickness or different chemical or phase composition.
Though TEM is a very useful tool for measuring the number of Cu precipitate, it is not widely used in spite of its excellent resolution and analytical capabilities. There are two reasons why the TEM is not widely used. The first reason is the difficulties involved in preparing the required thin samples. The second reason is the difficulties involved in correctly interpreting TEM images. Relating to the sample preparation problem, it needs to insure that the feature of interest is present with the sample region that has been thinned and prepared for TEM analysis. The most favorable TEM sample sections to VLSI studies are vertical cross-sections. It take several hours to prepare ion mill samples of the necessary thickness, and the preparation is an arduous task. According to the process mentioned above, it is hard to perform the quantitative analysis of copper precipitate in the prior art, because it is partially eliminated from the ion bombardment during the process of chemical plasma etching. In addition, the sample preparation is very complicated, it takes almost one day to examine the copper precipitate by TEM.